_Node is wrong type annotation for validateOverwriteExistingEntity

[srawlins]

I should not have used _Node. I didn't check with the analyzer, but it now says:

Analyzing lib...
  error • The argument type '(_DirectoryNode) → Null' can't be assigned to the parameter type '(_Node) → void' at lib/src/backends/memory/memory_directory.dart:76:9 • argument_type_not_assignable

[tvolkert]

But here, we call validateOverwriteExistingEntity(child), where child is a _Node...

Let's back up. _RenameOverwriteValidator is defined here like so:

typedef void _RenameOverwriteValidator<T extends _Node>(T existingNode);

So in renameSync, we want the validateOverwriteExistingEntity argument to just be an instance of _RenameOverwriteValidator, with the type argument being any subclass of _Node. How do we express this?